Disorders of attention are some of the most common disabilities that follow neurologic injury, and rehabilitation of attention is a critical component of a patient's long-term recovery of function. However, little is known about the cellular mechanisms underlying attention. The proposed studies take a systems approach to studying cognitive modulation of cortical processing by using the alteration of neuronal activity in the corticothalamic circuit with selective visual attention as a model case. The activity of both single neurons and small populations of neurons in visual cortex and in the pulvinar nucleus of the thalamus will be recorded during performance of a task that requires visual attention. Analysis of neural responses will provide quantitative information on the modulation of cortical function by attention. These observations will constrain the types of neural architectures capable of generating such modulation. The long-range goal is to use this information to develop rational therapeutic interventions.